1. Field of the Invention
The invention relates to a rolling method of a strip and a rolling mill of a sheet material which permits, upon rolling a strip, particularly upon cold-rolling a steel sheet or the like, improvement of the edge drop, and achievement of a uniform thickness distribution in the width direction throughout the entire width.
2. Description of the Related Art
From among thickness deviations in the width direction produced in a strip (material to be rolled) during rolling, a sharp thickness reduction at the both ends in the width direction is known as an edge drop. In order to obtain a satisfactory rolled product with a uniform thickness distribution (thickness profile) in the width direction by rolling, it is necessary to reduce the edge drop.
It is one of the conventional control practices for reducing the edge drop to cause work rolls (hereinafter sometimes abbreviated as "WR") having a tapered end on one side to shift in the axial direction.
Japanese Patent Publication No. 2-34,241 discloses a method comprising the steps of estimating a thickness profile on the exit side of a rolling mill from the thickness distribution in the width direction of the starting strip on the entry side of the rolling mill, distribution of roll gap between upper and lower work rolls, and the printing ratio of the roll gap distribution onto the rolled product, collating this estimated value with a target thickness profile, and causing the work rolls to shift to a position where the difference between the two values is minimum.
Japanese Patent Publication No. 2-4,364 discloses a technique for alleviating the edge drop, comprising the steps of using a pair of work rolls at least each of which has a converging tapered end on one side, locating the tapered portions at ends on the both sides during rolling, and improving the geometry of the roll gap at the ends on the both sides. This patent publication discloses also a case of application of this technique to a cold-rolling tandem mill, where at least a first stand is provided with the work rolls having the tapered portion.
Japanese Unexamined Patent Publication No. 60-12,213 discloses a method of performing a shift control of work rolls to adjust the shift position of the work rolls, comprising the steps of comparing and calculating an observed value and a target value of the quantity of edge drop by means of an edge drop meter installed on the exit side of a final stand and controlling shifting of the work rolls on the basis of the results of comparison and calculation.
Japanese Patent Publication No. 6-71,611 discloses a method of adjusting the quantity of shift of work rolls on the basis of a difference between an edge drop of a starting strip material for rolling before rolling as measured with an edge drop meter installed on the entry side of a rolling mill and a target value thereof, and a difference between an edge drop of a product after rolling as measured with an edge drop meter installed on the exit side of the rolling mill and a target value thereof.
Japanese patent Publication No. 2-34,241 discloses a method, proposed by the present applicant, of incorporating a thickness distribution in the width direction of a strip material to be rolled on the entry side of a rolling mill as a control factor. This method includes estimating a thickness distribution on the exit side of the rolling mill (final stand) or in a product, by means of a thickness distribution in the width direction of the strip material to be rolled before rolling, a distribution of the roll gap between upper and lower work rolls, and a printing ratio of this roll gap distribution onto the rolled product, and setting a shift position of the work rolls so as to achieve a minimum difference between this estimated value and a target thickness distribution.
References "Sheet Crown Edge Drop Control Characteristics" (the 45th Plastic Working Federation Lecture Meeting Preprint, pp. 403-406, 1994) and "Edge Profile Control Using Pair Cross Mill in Cold Rolling" (Iron and Steel engineer, pp. 20-26, June 1996) disclose findings that, by causing upper and lower work rolls to cross each other, together with backup rolls on respective sides, there is available an effect of achieving a uniform thickness profile (thickness distribution in the width direction) under the action of a parabolic roll gap produced from the width center toward the strip end between the upper and the lower work rolls.
As a technique of combining a roll crossing and a roll shifting for upper and lower work rolls, for example, Japanese Unexamined Patent Publication No. 57-200,503 discloses a technique comprising the steps, in a roll crossing rolling mill comprising groups of upper rolls and lower rolls crossing at a prescribed angle, of achieving a uniform wear of the work rolls, reducing the frequency of roll polishing, and thus improving the consumption of rolls by displacing the relative position of the work rolls from among the roll groups relative to the strip material to be rolled in an axial direction of rolls.
Japanese Unexamined Patent Publication No. 5-185,125 discloses a method of operating the roll shift and the work roll bending force in response to the changing timing of the roll crossing angle with a view to reducing the rejectable range of strip flatness produced in the course of changing the roll crossing angle, while changing set values of operating conditions during running along with passage by a coil welding point (strip joint).
In the methods disclosed in the foregoing Japanese Unexamined Patent Publication No. 2-4,364 and Japanese Patent Publication No. 2-34,241, the taper is imparted to the work rolls by polishing prior to rolling. It is therefore impossible to change the quantity of taper or the shape during rolling. Work rolls are not usually replaced for each coil, but are in service for rolling of several tens of coils. Upon continuous rolling of several tens of coils, increasing the quantity of taper imparted to the work rolls is effective for a coil having a large edge drop in the material strip. For a coil having a small edge drop in the material strip, however, an increased taper is not effective and an excessive thickness are produced near the inside of the strip ends in the width direction. A decreased taper is, in contrast, effective for a coil having a small edge drop in the material strip, whereas a decreased taper cannot sometimes ensure sufficient improvement for a coil having a large edge drop in the material strip. These methods have therefore a problem in that a uniform thickness profile is not available for the entire width through improvement of edge drop for all coils.
Japanese Unexamined Patent Publication No. 2-34,241 does not take account of the edge drop occurring behavior at stands in the downstream of a stand (control stand) having a roll shifting mechanism capable of changing the thickness distribution in the width direction, thus leading to a decrease in the estimation accuracy of thickness deviation in the width direction on the exit side of the final stand. When conducting rolling at a shift position of work rolls set by this method, there is posed a problem in that the thickness distribution in the width direction on the exit side of the final stand does not agree with a target thickness distribution.
In order to take the edge drop occurring behavior in the individual stands into account, it is necessary to measure the thickness deviation in the width direction on the exit side of each stand. In a cold tandem mill, however, the distance between stands is small, and further, there occurs splash of cooling water or lubricant oil. It is therefore difficult to install a sensor for measuring a thickness distribution in the width direction, which causes another difficulty of a high installation cost. In a tandem rolling mill, therefore, it is practically impossible to measure the thickness distribution in the width direction between stands during rolling.
In the method disclosed in the aforesaid reference "Sheet Crown edge Drop Control Characteristics," the roll gap slowly expands in a parabolic shape from the width center toward the strip end. While this brings about an effect of improving the so-called body crown (sheet crown), no effect can be expected in the reduction of an edge drop which is a thickness deviation at the end of width.
In the aforesaid Japanese Patent Publication No. 57-206,503 which has an object to prevent local wear of work rolls, it is impossible to control an edge drop.
The technique disclosed in the aforesaid Japanese Unexamined Patent Publication No. 5-185,125 has an object to prevent deterioration of a strip shape during the transition period for changing the crossing angle. A problem here is that an improvement effect of edge drop over that of the technique disclosed in the foregoing Japanese Unexamined Patent Publication No. 2-4,364 cannot be expected from this technique.